The present invention relates generally to recovering information from a data stream, and more particularly to recovering clock and data information from a serial data stream.
In digital systems data is often transferred from a transmitter to a receiver. Data is sent from the transmitter to the receiver at a nominal rate. To maximize the data sent during a given period for a given bandwidth, a clock signal for the data is generally not sent directly in the transmission. Instead the clock signal is regenerated by the receiver using the received data. In general, a clock recovery unit (CRU) in the receiver generates a clock signal, with the CRU being based on a phase locked loop (PLL), delay locked loop (DLL), or similar device.
Using a PLL as an example, a PLL is often employed in CRUs to align a locally generated clock signal relative to a phase and frequency of received data. The PLL adjusts operation of an oscillator of the PLL to form a clock signal locked in phase and frequency to that of the received data signal. Acquisition time, or lock time, refers to the amount of time the PLL requires for convergence within a certain phase error of the received signal, i.e., the time required for the PLL to go from an initial frequency offset to proper clock alignment. Typically lock times are on the order of hundreds to thousands of bit times. The amount of lock time taken by a particular CRU depends on the device employed.
For example, a CRU in a digital communication system may require reception between about 300 to about 1500 bits in order to lock to a data stream. These 300 to 1500 “lock bits” are generally effectively lost from the data stream, thereby reducing the effective bandwidth of the transmission medium. If the number of bits for any particular transmission stream is relatively small, a significant loss in transmission efficiency may occur. For example, using a relatively short data transmission packet, such as about 3K Bytes (24,000 Bits), losing 300 to 1500 bits results in a 1.25% to 6.25% loss of transmission efficiency. Moreover, when the input serial data becomes highly distorted, for example, the data develops a duty cycle of less than about 30%, a data-eye of the received signal may become small enough to potentially disrupt the operation of the CRU.